1. Field of the Invention
The invention in general, relates to sonar systems, and particularly to a side-looking multi-beam sonar system.
2. Description of the Prior Art
In side-looking sonar systems apparatus on a carrier vehicle repetitively transmits acoustic pulses (pings) to sonify the target area and energy reflected from adjacent narrow strips on the target area is picked up by a multi-segmented elongated transducer and is portrayed as a line-by-line picture that is a pattern of highlights and shadows analogous to an optically viewed panorama illuminated by side-lighting, with objects outlined in such a way as to permit their identification.
By increasing the area detected and portrayed for each transmitted pulse, the carrier speed and therefore the search or mapping rate is significantly increased. This is accomplished by the use of multiple receiver beams such as described in U.S. Pat. No. 3,950,723. In that Patent, which is herein incorporated by reference, the individual output signals from the plurality of transducer segments of the elongated receiving transducer are processed in such a manner as to form multiple beams and the apparatus is operable such that the focus is electronically varied with range so that any and all returns are constantly in focus.
There is considerable interest in applying the principles of synthetic aperture side-looking radar to side-looking sonar. A synthetic aperture system is based on the generation of an effectively long antenna or receiver transducer by signal processing means rather than by the actual use of a physically long antenna or transducer. In a synthetic aperture side-looking sonar system the receiver transducer is moved to take up sequential positions along a line and during movement acoustic transmissions take place and the reflected energy is evidenced by the transducer segment output signals which are stored. After a predetermined number of sequential pings during the course of travel, all of the stored signals are suitably processed to form one or more synthetic aperture beams each having a higher resolution, due to the longer aperture, than the real aperture beam.
Depending upon the number of transducer segments of the receiver transducer, such system requires a relatively large amount of storage capability and if the apparatus is located on a deep towed vehicle, all of this storage information must be transmitted up the tow cable thus requiring either multiplexing equipment or larger capacity cables.
In addition, although the synthetic aperture sonar provides better resolution, there are situations where resolution can be sacrificed for greater speeds. In the present invention, the apparatus can operate in the real aperture mode as described in the mentioned patent for faster carrier speeds and can additionally be operated in a synthetic aperture mode to obtain greater resolution.